The present application concerns photovoltaic devices, such as solar cell devices. More specifically, the present application concerns photovoltaic devices with backside contact to a buried semiconductor layer, such as an emitter layer.
A photovoltaic device converts light energy into electricity. Although the term “solar cell device” may sometimes be used to refer to a device that captures energy from sunlight, the terms “solar cell device” and “photovoltaic device” are interchangeably used in the present application regardless of the light source.
FIG. 1 is a cross-sectional view of a conventional multi-junction solar cell device 100. The multi-junction solar cell device 100 may include multiple junction regions, such as the first p-n junction region 110, the second p-n junction region 120, and the third p-n junction region 130, which are connected in series. Each p-n junction region may contain multiple layers including an emitter layer and a base layer (not shown).
The multi-junction solar cell device 100 may receive light from the front or top (illuminated) side of the device. The multi-junction solar cell device 100 may include emitter “grid” contacts 140 on the front or top side of the device 100, which may contact the emitter layer of the first junction region 110. The multi-junction solar cell device 100 may also include base contacts 150 on the backside or bottom (non-illuminated) surface of the device 100, which may contact the base layer of the third junction region 130.
Electrical connection to the device 100 may be made through the emitter “grid” contacts 140 and the base contacts 150. The base contacts 150 may cover the entire backside surface of the device 100, while the emitter “grid” contacts 140 may consist of an array of fingers or buses that collect current from the top or front side of the device 100. An antireflection coating 160 may be deposited over the exposed top surface of the emitter layer of the first junction region 110 to minimize losses due to reflections.
In the conventional multi-junction solar cell device 100, the area shadowed by the top grid contact 140 may cause a significant loss in device efficiency, because no light is absorbed in the shadowed area. The metal fingers of the top grid contact 140 may cover at least 5% of the top surface of the device 100, which is significant. Conventional methods for minimizing the shadowed area are extremely limited due to the performance trade-off between the shadowed area and associated resistive losses. In other words, reducing the area of grid metal covering the surface of the solar cell device 100 may decrease the shadow losses, but it increases the series resistance in the device 100. Particularly in high current applications, such as optical concentrator systems, this series resistance becomes a key factor that limits overall device efficiency.
Accordingly, a new solar cell structure is needed for reducing shadow losses without unnecessarily increasing the series resistance of the solar cell device.